MakerKiln

Belt Length Calculator

The belt length for a two-pulley drive, from the pulley diameters and the center distance, with the tooth count for a timing belt.

Local onlyThe math runs in JavaScript in this tab. Your figures are remembered in this browser's local storage, and nothing is uploaded.

A timing belt around two pulleys has to be just the right length, snug without straining the bearings. This works it out from the two pulley diameters and the distance between their centers, and turns it into a tooth count so you can order the right GT2 or HTD belt.

Pitch diameter, not the edge

Measure, or look up, the pitch diameter of each pulley, which is where the belt's teeth actually sit, not the outside of the flanges. For GT2 it is the tooth count times 2 divided by pi, so a 20-tooth pulley is 12.73 mm. Using the outer diameter throws the length off by a few millimeters, which is enough to matter on a closed loop.

The length and the teeth

The belt wraps half of each pulley plus the two straight runs between them, with a small correction when the pulleys are different sizes. Divide that length by the belt pitch and you have the tooth count. GT2, the printer standard, is a 2 mm pitch, so a 400 mm belt is 200 teeth. HTD and T5 use different pitches, so set the pitch to match your belt.

Closed loop or cut to length

Closed-loop belts come in set tooth counts, so pick the nearest one at or just under your figure and let the pulley spacing or a tensioner take up the slack. For an axis that clamps both belt ends to a carriage, like a printer's X or Y, you buy open-ended belt off a roll and cut it, and the exact standard length does not matter.

Getting the tension right

Aim for snug, not tight. A belt stretched hard loads the pulley bearings and the motor shaft and wears everything faster; a slack belt skips teeth and loses position. Set the center distance from this length, then fine-tune with a tensioner until the belt is firm and plucks with a low, clean note.

More MakerKiln tools

Put this calculator on your site

Free to embed, and welcome. It runs the same way, in your visitor's browser, and it sizes itself to fit. Paste this where you want it, and it stays current on its own:

<iframe src="https://makerkiln.com/belt-length-calculator/embed/" title="Belt Length Calculator" loading="lazy" style="width:100%;max-width:560px;height:560px;border:1px solid #d9dee8;border-radius:14px"></iframe>
<p style="font:13px/1.4 system-ui,sans-serif;margin:6px 2px">Free calculator by <a href="https://makerkiln.com/belt-length-calculator/" target="_blank" rel="noopener">MakerKiln</a></p>
<script>window.addEventListener("message",function(e){if(e.data&&e.data.mk==="h"&&e.data.slug==="belt-length-calculator"){var f=document.querySelector('iframe[src="https://makerkiln.com/belt-length-calculator/embed/"]');if(f){f.style.height=e.data.h+"px"}}});</script>
Download the file

Prefer to host it yourself, offline or on an intranet? Download the file above. Keep the MakerKiln credit and it is free to use.